Carbon is known to have four unique crystalline structures, including diamond, graphite, fullerene, and carbon nano-tubes (and its larger-diameter cousins—carbon nano-fibers or CNFs). The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall, which can be conceptually obtained by rolling up a graphene sheet or several graphene sheets to form a concentric hollow structure. A graphene sheet or basal plane is composed of carbon atoms occupying a two-dimensional hexagonal lattice. Carbon nano-tubes have a diameter on the order of a few nanometers to a hundred nanometers. Carbon nano-tubes can function as either a conductor or a semiconductor, depending on the rolled shape and the diameter of the tubes. Its longitudinal, hollow structure imparts unique mechanical, electrical and chemical properties to the material. Carbon nano-tubes are believed to have great potential for use in field emission devices, hydrogen fuel storage, rechargeable battery electrodes, and as composite reinforcements.
However, CNTs are extremely expensive due to the low yield and low production rates commonly associated with all of the current CNT preparation processes, such as arc discharge, laser ablation, chemical vapor deposition (CVD), and catalytic CVD (CCVD). The high material costs have significantly hindered the widespread application of CNTs. Earlier CNT production methods include those disclosed in the following patents: H. G. Tennent, “Carbon Fibrils, Method for Producing Same and Compositions Containing Same,” U.S. Pat. No. 4,663,230 (May 5, 1987); C. Snyder, “Carbon Fibrils,” U.S. Pat. No. 5,707,916 (Jan. 13, 1998).
Carbon nano-fibers (CNFs) are prepared from CVD, CCVD, or electro-spinning of polymer nano-fibers followed by carbonization. Electro-spinning has not been regarded as a mass-production method due to the limited amount of material that can be electro-spun with one hollow needle head. An example of the process to produce polymer nano-fibers via electro-spinning is given in D. H. Reneker, et al, “Processes for Producing Fibers and Their Use,” US Pub. No. 2009/0039565 (Feb. 12, 2009).
The CNFs produced by the CVD and CCVD processes are commonly referred to as vapor-grown carbon nano-fibers (VG-CNFs). VG-CNFs have been extensively investigated in recent years and are commercially available at very high prices (e.g., $300/Kg). The following are some examples of CNF production processes: D. J. C. Yates, et al., “Production of Carbon Filaments,” U.S. Pat. No. 4,565,683 (Jan. 21, 1986); S. H. Yoon, “Ultra-fine Fibrous Carbon and Preparation Method Thereof,” US Pub. No. 2009/0075077 (Mar. 19, 2009); S. H. Yoon, “Ultra-fine Fibrous Carbon and Preparation Method Thereof,” U.S. Pat. No. 7,470,418 (Dec. 30, 2008); S. H. Yoon, “Porous Filamentous Nano Carbon and Method of Forming the Same,” US Pub. No. 2009/0004095 (Jan. 1, 2009); G. Oriji, “Carbon Nano Fiber, Production and Use,” US Pub. No. 2009/0008611 (Jan. 8, 2009); J. L. Gonzales Moral, et al., “Carbon Nanofibers and Procedure for Obtaining Said Nanofibers,” US Pub. No. 2009/0035569 (Feb. 5, 2009).
VG-CNFs and related CNTs have several drawbacks that have significantly constrained their scope of application:                (a) Both CVD and CCVD processes typically involve using a catalyst and the catalyst particles (e.g., transition metal nano particles or their alloys) usually become part of the resulting CNF or CNT structure. Normally, there is a significant amount of catalyst used in these processes. The residual catalyst, even just a trace amount, is considered undesirable in many applications. For instance, Fe is viewed as detrimental to the performance of a lithium ion battery if CNFs or CNTs are used as an anode active material. Catalytic particles can also catalyze or accelerate thermal or chemical degradation of a polymer matrix composite material.        (b) The CVD or CCVD processes intrinsically introduce a significant amount of impurities into the resulting CNFs or CNTs. It is not unusual to find a purity level (graphitic carbon content) in a CNF less than 80-90%.        (c) Depending upon the processing conditions, the graphene planes in different CNFs may be oriented at different angles with respect to the fiber axis. Furthermore, the graphene planes may be curved as a cup-shape or a cone-helix structure, which are not conducive to achieving high strength or modulus along the fiber axis. In one example, the fibers consist primarily of conical nano-fibers, but can contain a significant amount of bamboo nano-fibers. Most conical nano-fibers consist of an ordered inner layer and a disordered outer layer. When subjected to a thermal treatment above 1,500° C., some CNFs can undergo a structural transformation with the ordered inner layers changing from a cone-helix structure to a highly ordered multiwall stacked cone structure. The bamboo nano-fibers can have a tapered multiwall nanotube structure for the wall and a multi-shell fullerene structure for the cap of each segment, surrounded by a disordered outer layer. When these fibers are heat treated, the disordered outer layers transform to an ordered multiwall nanotube structure and merge with the wall of each segment. The end caps of each segment transform from a smooth multiwall fullerene structure to one consisting of disjointed graphene planes. Such a thermally induced instability in the CNF structure is an undesirable feature of CNFs for high-temperature applications (e.g., as a reinforcement in a carbon matrix composite).        (d) The CNFs typically have a continuous thermal carbon overcoat, which is a result of the thermal decomposition effect during the CNF formation process via the CVD, CCVD, or carbonization of electro-spun polymer nano-fibrils. Although this carbon overcoat could serve as a protective layer for the internal graphitic crystallites in some applications, the overcoat is detrimental to many other engineering applications. For instance, this overcoat makes it difficult to chemically functionalize the CNF surface, thereby inhibiting the formation of a strong bond between a CNF and a polymer matrix in a polymer composite. In a similar manner, a CNT has a complete, continuous graphene plane wrapped around the tube axis, which has few active sites where chemical functionalization can occur. Hence, chemical functionalization occurs only at the edge unless this surface is chemically treated (e.g., with a strong oxidizing agent, such as fuming sulfuric acid and nitric acid).        (e) In most of the VG-CNFs, the graphene planes or graphitic crystallites are oriented at a non-zero angle with respect to the fiber axis, resulting in a lower strength, modulus, thermal conductivity, and electrical conductivity along the fiber axis direction as compared with fibrils having all graphene planes substantially parallel to the fiber axis (e.g., CNTs).        
Hence, it is desirable to have a carbon or graphite nano-fiber that has a well-controlled, consistent, and stable structure to ensure consistent properties and performance. It is further desirable to have a low-cost process that is capable of producing CNFs or graphitic nano-fibers in large quantities. It is also desirable to have CNFs or graphitic nano-fibers that are pure and catalyst-free. It is still further desirable to have CNFs or graphitic nano-fibers that exhibit much more surface areas for chemical functionalization or interactions with a chemical species or a matrix material. It is most desirable to have a CNF or graphitic fibril that exhibits a higher strength, modulus, thermal conductivity, and electrical conductivity as compared with conventional CNFs. The main object of the present invention is to provide submicron- or nanometer-scaled graphitic fibers that exhibit these desirable attributes.